One wireless communication technology that is drawing attention in recent years is the IEEE 802.16. As an alternative of, for example, telephone lines and optical fiber lines, IEEE 802.16 has been developed as a method of building a Wireless MAN (Metropolitan Area Network) which is a wide area network for wirelessly connecting carriers and a user's home and connecting between LANs (Local Area Network) of urban areas and specific areas. IEEE 802.16 can cover an area having a radius of approximately 50 km with a maximum transmission rate of approximately 70 megabits/second.
Currently, in the IEEE 802.16 working group, a 802.16d specification (see non-patent document 1) for fixed communications and a 802.16e specification (see non-patent document 2) for mobile communications are mainly being standardized.
In the WiMAX (Worldwide Interoperability for Microwave Access) Forum which is an organization for ensuring connection among communication devices and systems based on the 802.16d/e specifications, a FFR (Fractional Frequency Reuse) is proposed as one form of reusing wireless frequencies of a wireless communication system based on 802.16d/e (see non-patent document 3).
In one representative example of reusing frequencies used by each cell in a wireless communication system, there is a 3 frequency repetition method (hereinafter referred to as “Reuse 3”) of dividing available frequencies F1, F2, F3 into three parts and exclusively using each frequency at cell 1, cell 2, and cell 3 (as illustrated in FIG. 1). As another example, there is a 1 frequency repetition method (hereinafter referred to as “Reuse 1”) of allowing shared use of available frequencies with each cell (as illustrated in FIG. 2).
In a case where of further dividing each cell into 3 sectors, the same can be said where a cell is replaced with a sector. Nevertheless, the below-description uniformly uses the term “cell”.
In a case where available frequencies are the same, the maximum throughput of each cell with Reuse 1 is 3 times compared to that of Reuse 3. On the other hand, in terms of the influence of interference among the cells, the cell 1, for example, receives interference from adjacent cells (cell 2 and cell 3) because Reuse 1 allows each cell to use the same frequency. Particularly, a wireless terminal (hereinafter referred to as MS (Mobile Station)) is affected greater the closer to a boundary area between cells.
On the other hand, with Reuse 3, the cell 1, for example, is unaffected by interference because adjacent cells 2 and 3 use different frequencies. In a case where there is cell 1′ (not illustrated) having cell 2 interposed between cell 1 and using a preceding frequency F1, the attenuation of interference waves from the cell 1′ becomes greater towards the cell 1 because the cell 1′ is located farther compared to adjacent cells 2 and 3. Thus, the influence of interference is significantly smaller compared to that of Reuse 1. That is, with Reuse 1, a MS located in the vicinity of a cell boundary experiences degradation of channel quality and difficulty in communicating due to interference waves from adjacent cells. Therefore, coverage of the MS becomes small with Reuse 1. Reuse 3, however, faces no such problems. Therefore, coverage of the MS can be expanded.
With the above-described FFR, adjacent base stations use different frequencies at a first time period but are allowed to share a frequency in a second time period. In this example, by combining the advantages of Reuse 1 and Reuse 3, a wide coverage as Reuse 3 can be maintained while throughput of the entire system is improved to be as close as possible to that of the Reuse 1.
FIG. 3 illustrates an example of an uplink sub-frame of an OFDMA wireless frame in a case where FFR is applied. Although control data regions such as a preamble, a FCH (Frame Control Header), a DL-MAP, a UL-MAP exist in the frame in a case of IEEE 802.16d/e, the control data regions are omitted from this drawing.
Further, although the frame is divided in a downlink (downward direction from a wireless base station to a wireless terminal) sub-frame and an uplink (upward direction from a wireless terminal to a wireless base station) sub-frame in a case of TDD (Time Division Duplexing), only a single configuration is illustrated for the sake of simplification. The horizontal axis represents a time direction with symbols serving as units (modulation is performed once for a single symbol), and the vertical axis represents a frequency direction with sub-channels serving as units (frequency group formed of plural sub-carriers). As illustrated in FIG. 3, the sub-frame can be divided into two parts in the time direction, for example. One may be used as a Reuse 3 zone and the other may be used as a Reuse 1 zone. In the Reuse 3 zone, an available sub-channel is divided into three parts F1, F2, F3 in the frequency direction and is used exclusively in 3 cells. In the Reuse 1 zone, the sub-channel is shared. The manner in which frequencies of cells 1-3 are used in this state is illustrated with FIG. 4. In the time period using Reuse 3, the cells 1, 2, and 3 each exclusively use F1, F2, and F3 and can cover an MS located in the vicinity of a cell boundary. In the time period using Reuse 1, an MS is mainly located in the vicinity of a center of a cell at which communication can be performed even where there is interference from adjacent cells.    Non-patent document 1: IEEE Std 802.16™-2004    Non-patent document 2: IEEE Std 802.16e™-2005    Non-patent document 3: Mobile WiMAX—Part I: A Technical Overview and Performance Evaluation (August 2006).
With the above-FFR, it is possible to achieve wide coverage and high throughput. Further, FFR can be applied to a downlink from a wireless base station (hereinafter referred to as “BS” (Base Station) to a MS and to an uplink from a MS to a BS. Particularly, a special control region is required to be provided in the uplink of IEEE 802.16d/e for feedback data and contention access from the MS to the BS. To that extent, available resources for enabling user data transmission of the MS decrease. Therefore, it is an important task to improve throughput more in the uplink than the downlink.
However, in a case of applying FFR to the uplink, the following problems arise. In a case of the uplink, a MS which is located far from a cell center requires to transmit uplink wireless signals with large transmission power even in a case where a Burst Profile (a transmission parameter indicating a combination of a modulation scheme and an encoding scheme, hereinafter referred to as “BP”) used for transmitting uplink data is the same. Even for MS having the same distance from a cell center, a large transmission power is required in order to use a high speed BP for transmitting large amounts of uplink data. In a case of a downlink, a base station, which is the source of transmitting wireless signals, is located significantly apart from other base stations. Further, a MS located in the vicinity of a cell center is less likely to be affected by interference from adjacent cells even with Reuse 1. Therefore, wireless channel quality of the base station is relatively high. Thus, downlink data can be transmitted using high speed BP.
On the other hand, in a case of an uplink, a MS, which is the source of transmitting wireless signals, tends to have a short distance with respect to a base station(s) other than the base station performing wireless communications with the MS when compared with the distances between base stations. Further, the faster BP is used by the MS, the greater transmission power becomes. To that extent, interference to adjacent cells using the same frequency becomes greater.
FIG. 5 illustrates the manner in which interference occurs during uplink frame transmission. MS1 and MS2 are connected to cell 1 and cell 2, respectively. The distance away from the cell center (BS) is greater for MS2. In this example, MS1 and MS2 both use Reuse 1 for communicating with each other. MS1 uses QPSK 1/2 (a modulation scheme of QPSK, an encoding scheme having error correction code percentage of 50%). MS2 uses 16 QAM 1/2 (a modulation scheme of 16 QAM, an encoding scheme having error correction code percentage of 50%).
MS2 is located farther from the BS (counterpart) compared to MS1. Further, MS2 uses a high speed BP. Therefore, MS2 needs to transmit uplink wireless signals with greater transmission power. Accordingly, the transmission signals of MS2 travels to adjacent cells 1 and 3 in the form of interference waves. Thereby, in a case where a BS of cell 1 receives transmission signals from a MS1 which also uses Reuse 1, the reception quality of the MS1 is degraded. In a case of the uplink, degradation of wireless channel quality occurs, and throughput of the entire system decreases.